Yellow fever virus (YFV) causes the mosquito-borne disease yellow fever (YF), a viscerotropic disease, i.e., it targets the liver, of primates. The disease is controlled by a very efficacious live attenuated vaccine, strain 17D, which was derived from wild-type strain Asibi and has been administered to over 540 million people in the last 70 years. In the last 10 years a rare, but fatal, condition has been reported due to the vaccine, termed YF vaccine associated viscerotropic disease (YEL-AVD). This condition is associated with uncontrolled replication of the virus in primary vaccinees resulting in pansystemic infection and a disease picture very similar to wild-type YFV. Originally, YEL-AVD was reported to have an incidence of 0.3 per 100,000 vaccinees. However, in October 2007, an incidence of 9 per 100,000 was reported in Peru. This alarming rate of serious adverse events is causing regulatory authorities to re-evaluate the contraindications and use of the 17D vaccine, and a number of individuals are calling for the development of a new YF vaccine. Nonetheless, it is recognized that development of a new vaccine takes at least 15 years. Thus, there is an urgent need to undertake research on the current vaccine to understand YEL-AVD and how the current vaccine could be improved. This urgency is emphasized by the use of 17D vaccine virus to generate chimeric viruses containing the structural protein genes of one flavivirus (e.g. Japanese encephalitis) in a 17D vaccine virus backbone. Such chimeric vaccines are in phase II clinical trials. Surprisingly, little is known about the molecular mechanisms that govern the virulence of wild-type YFV or the attenuation and immunogenicity of the live- attenuated YFV 17D vaccine. This is, in part, due to the lack of a small animal of viscerotropism. The PI has developed a new YFV infection model that readily distinguishes wild-type Asibi strain from the 17D vaccine strain. Our long-term goals are to use this model to understand the immunologic basis of viscerotropism and attenuation. Our specific aims are: i) investigate the steps in the pathogenic sequence at which attenuated 17D virus infection is blocked compared with virulent Asibi virus thereby identifying potential anomalies in the innate immune response that might be important to the development of YEL-AVD; and ii) identify YFV-encoded molecular determinants of viscerotropism present in YEL-AVD isolates by examining their viscerotropic phenotype in the new mouse model developed in this project. PUBLIC HEALTH RELEVANCE: The highly-lethal viral hemorrhagic fever caused by the mosquito-borne yellow fever virus (YFV) was one of the most feared diseases in Africa, Europe and the Americas until the live-attenuated 17D vaccine was developed in the 1930's. Even today, over 200,000 West Africans contract YF annually, with tens of thousands of fatalities. The attenuated 17D vaccine strain was derived by repeatedly growing a wild- type YFV isolate (strain Asibi) in cultured cells. Although 17D is considered to be one of the most effective live-attenuated virus vaccines ever developed, the immunologic mechanisms that control the attenuation and immunogenicity of this live-attenuated vaccine remain a mystery. Our long-term goal is to determine how the host is able to control the 17D infection and investigate mechanisms by which vaccine-associated viscerotropic disease (YEL-AVD) occurs. To achieve this goal, Drs. Ryman and Barrett have proposed a consortium agreement in which Dr. Barrett's laboratory will provide viruses to Dr. Ryman's laboratory, where their virulence will be assessed using a newly developed model of YFV pathogenesis and disease. Our understanding of host- pathogen interactions has increased sufficiently to allow rational design of live- attenuated virus strains and the technology exists to introduce and test mutations in genetically-engineered vector systems. It is anticipated that our findings will improve the safety and efficacy of the YFV vaccine, and additionally facilitate the rational design of other live-attenuated virus vaccines, particularly against other pathogenic flaviviruses (e.g., West Nile and dengue viruses) and the closely-related alphaviruses (e.g., eastern equine encephalitis virus), most of which are agents of both emerging infectious disease and bioterrorism/biowarfare.